在介绍 Angular Injector (注入器) 之前,我们先要了解 Dependency Injection,即依赖注入的概念。
依赖注入允许程序设计遵从依赖倒置原则 (简单的说就是要求对抽象进行编程,不要对实现进行编程,这样就降低了客户端与实现模块间的耦合) 调用者只需知道服务的接口,具体服务的查找和创建由注入器 (Injector) 负责处理并提供给调用者,这样就分离了服务和调用者的依赖,符合低耦合的程序设计原则。
从上述的内容可知,依赖注入中包含三种角色:调用者、服务和注入器 (Injector)。现在我们开始介绍 Injector,在 Angular 中 Injector (注入器) 用来管理服务对象的创建和获取。接下来我们先来看一下 Injector 抽象类:
Injector 抽象类
// angular2\packages\core\src\di\injector.ts export abstract class Injector { static THROW_IF_NOT_FOUND = _THROW_IF_NOT_FOUND; static NULL: Injector = new _NullInjector(); /** * 用于根据给定的Token从注入器中获取相应的对象。 * 如果没有找到相应的对象,将返回notFoundValue设置的值。若notFoundValue的值与 * _THROW_IF_NOT_FOUND相等,则会抛出异常。 */ abstract get<T>(token: Type<T>|InjectionToken<T>, notFoundValue?: T): T; } const _THROW_IF_NOT_FOUND = new Object();
Injector 抽象类中定义了一个 get() 抽象方法,该方法用于根据给定的 Token 从注入器中获取相应的对象,每个Injector 抽象类的子类都必须实现该方法。在 Angular 中常见的 Injector 抽象类子类有:
下面我们来依次介绍它们:
_NullInjector 类
_NullInjector 类的实例用于表示空的注入器。
// angular2\packages\core\src\di\injector.ts class _NullInjector implements Injector { get(token: any, notFoundValue: any = _THROW_IF_NOT_FOUND): any { if (notFoundValue === _THROW_IF_NOT_FOUND) { throw new Error(`No provider for ${stringify(token)}!`); } return notFoundValue; } }
ReflectiveInjector 抽象类
ReflectiveInjector 表示一个依赖注入容器,用于实例化对象和解析依赖。
ReflectiveInjector 使用示例
@Injectable() class Engine {} @Injectable() class Car { constructor(public engine:Engine) {} } var injector = ReflectiveInjector.resolveAndCreate([Car, Engine]); var car = injector.get(Car); expect(car instanceof Car).toBe(true); expect(car.engine instanceof Engine).toBe(true);
上面示例中,我们通过调用 ReflectiveInjector 抽象类的 resolveAndCreate() 方法,创建注入器。然后通过调用注入器的 get() 方法,获取 Token 对应的对象。该抽象类除了 resolveAndCreate() 静态方法外,还含有以下静态方法:
接下来我们来分析上述的静态方法:
resolveAndCreate()
static resolveAndCreate(providers: Provider[], parent?: Injector): ReflectiveInjector { const ResolvedReflectiveProviders = ReflectiveInjector.resolve(providers); return ReflectiveInjector.fromResolvedProviders(ResolvedReflectiveProviders, parent); }
从上面代码中,我们可以看出 resolveAndCreate() 方法内部是通过调用 ReflectiveInjector.resolve() 方法和 ReflectiveInjector.fromResolvedProviders() 方法来创建 ReflectiveInjector 对象。
resolve()
该方法用于把 Provider 数组解析为 ResolvedReflectiveProvider 数组。
static resolve(providers: Provider[]): ResolvedReflectiveProvider[] { return resolveReflectiveProviders(providers); }
resolve() 使用示例
@Injectable() class Engine {} @Injectable() class Car { constructor(public engine:Engine) {} } var providers = ReflectiveInjector.resolve([Car, [[Engine]]]); expect(providers.length).toEqual(2); expect(providers[0] instanceof ResolvedReflectiveProvider).toBe(true); expect(providers[0].key.displayName).toBe("Car"); expect(providers[1].key.displayName).toBe("Engine");
resolve() 解析图示
Provider 类型
export type Provider = TypeProvider | ValueProvider | ClassProvider | ExistingProvider | FactoryProvider | any[]; // ApiService export interface TypeProvider extends Type<any> {} // { provide: ApiService, useClass: ApiService } export interface ClassProvider { // 用于设置与依赖对象关联的Token值,Token值可能是Type、InjectionToken、OpaqueToken的实例或字符串 provide: any; useClass: Type<any>; // 用于标识是否multiple providers,若是multiple类型,则返回与Token关联的依赖对象列表 multi?: boolean; } // { provide: 'API_URL', useValue: 'http://my.api.com/v1' } export interface ValueProvider { provide: any; useValue: any; multi?: boolean; } // { provide: 'ApiServiceAlias', useExisting: ApiService } export interface ExistingProvider { provide: any; useExisting: any; multi?: boolean; } // { provide: APP_INITIALIZER, useFactory: configFactory, deps: [AppConfig], multi: true } export interface FactoryProvider { provide: any; useFactory: Function; deps?: any[]; // 用于设置工厂函数的依赖对象 multi?: boolean; }
ResolvedReflectiveProvider 接口
export interface ResolvedReflectiveProvider { // 唯一的对象用来从ReflectiveInjector中获取对象 key: ReflectiveKey; // 工厂函数用于创建key相关的依赖对象 resolvedFactories: ResolvedReflectiveFactory[]; // 标识当前的provider是否为multi-provider multiProvider: boolean; }
ResolvedReflectiveFactory 类
export class ResolvedReflectiveFactory { constructor( public factory: Function, public dependencies: ReflectiveDependency[]) {} }
ReflectiveDependency 类
export class ReflectiveDependency { constructor( public key: ReflectiveKey, public optional: boolean, public visibility: Self|SkipSelf|null) {} static fromKey(key: ReflectiveKey): ReflectiveDependency { return new ReflectiveDependency(key, false, null); } }
ReflectiveKey 类
ReflectiveKey 对象中包含两个属性:系统范围内唯一的id 和 token。系统范围内唯一的id,允许注入器以更高效的方式存储已创建的对象。另外我们不能手动的创建 ReflectiveKey,当 ReflectiveInjector 对象解析 providers 的时候会自动创建 ReflectiveKey 对象。
export class ReflectiveKey { constructor(public token: Object, public id: number) { if (!token) { throw new Error('Token must be defined!'); } } // 返回序列化的token get displayName(): string { return stringify(this.token); } // 获取token对应的ReflectiveKey static get(token: Object): ReflectiveKey { return _globalKeyRegistry.get(resolveForwardRef(token)); } // 获取系统中已注册ReflectiveKey的个数 static get numberOfKeys(): number { return _globalKeyRegistry.numberOfKeys; } } const _globalKeyRegistry = new KeyRegistry(); // 创建Key仓库 export class KeyRegistry { private _allKeys = new Map<Object, ReflectiveKey>(); /** * 若token是ReflectiveKey类的实例,则直接返回。若_allKeys对象中包含token属性 * 则返回token对应的ReflectiveKey对象。否则创建一个新的ReflectiveKey对象,并 * 保存到_allKeys对象中 */ get(token: Object): ReflectiveKey { if (token instanceof ReflectiveKey) return token; if (this._allKeys.has(token)) { return this._allKeys.get(token) !; } const newKey = new ReflectiveKey(token, ReflectiveKey.numberOfKeys); this._allKeys.set(token, newKey); return newKey; } // 获取已保存ReflectiveKey的个数 get numberOfKeys(): number { return this._allKeys.size; } }
分析完 resolve() 方法的输入参数和返回类型,我们来看一下该方法内部的具体实现:
export function resolveReflectiveProviders(providers: Provider[]) : ResolvedReflectiveProvider[] { const normalized = _normalizeProviders(providers, []); // 步骤一 const resolved = normalized.map(resolveReflectiveProvider); // 步骤二 const resolvedProviderMap = mergeResolvedReflectiveProviders(resolved, new Map()); // 步骤三 return Array.from(resolvedProviderMap.values()); // 步骤四 }
步骤一 —— 规范化Provider
const normalized = _normalizeProviders(providers, []); // 规范化Providers function _normalizeProviders(providers: Provider[], res: Provider[]): Provider[] { providers.forEach(b => { // providers: [Type] => providers: [{provide: Type, useClass: Type }] if (b instanceof Type) { res.push({provide: b, useClass: b}); } else if (b && typeof b == 'object' && (b as any).provide !== undefined) { res.push(b as NormalizedProvider); } else if (b instanceof Array) { // 若b是数组,则递归调用_normalizeProviders()方法 _normalizeProviders(b, res); } else { throw invalidProviderError(b); } }); return res; } interface NormalizedProvider extends TypeProvider, ValueProvider, ClassProvider, ExistingProvider, FactoryProvider {}
步骤二 —— 转化NormalizedProvider为ResolvedReflectiveProvider
const resolved = normalized.map(resolveReflectiveProvider); // 解析NormalizedProvider为ResolvedReflectiveProvider function resolveReflectiveProvider(provider: NormalizedProvider): ResolvedReflectiveProvider { return new ResolvedReflectiveProvider_( ReflectiveKey.get(provider.provide), [resolveReflectiveFactory(provider)], provider.multi || false); } // 用于创建已解析的Provider实例 export class ResolvedReflectiveProvider_ implements ResolvedReflectiveProvider { constructor( public key: ReflectiveKey, public resolvedFactories: ResolvedReflectiveFactory[], public multiProvider: boolean) {} get resolvedFactory(): ResolvedReflectiveFactory { return this.resolvedFactories[0]; } } // 解析NormalizedProvider对象,创建ResolvedReflectiveFactory对象 function resolveReflectiveFactory(provider: NormalizedProvider): ResolvedReflectiveFactory { let factoryFn: Function; let resolvedDeps: ReflectiveDependency[]; if (provider.useClass) { // { provide: ApiService, useClass: ApiService } const useClass = resolveForwardRef(provider.useClass); factoryFn = reflector.factory(useClass); resolvedDeps = _dependenciesFor(useClass); } else if (provider.useExisting) { // { provide: 'ApiServiceAlias', useExisting: ApiService } factoryFn = (aliasInstance: any) => aliasInstance; resolvedDeps = [ReflectiveDependency.fromKey(ReflectiveKey.get(provider.useExisting))]; } else if (provider.useFactory) { // { provide: APP_INITIALIZER, useFactory: configFactory, deps: [AppConfig], // multi: true } factoryFn = provider.useFactory; resolvedDeps = constructDependencies(provider.useFactory, provider.deps); } else { // { provide: 'API_URL', useValue: 'http://my.api.com/v1' } factoryFn = () => provider.useValue; // const _EMPTY_LIST: any[] = []; resolvedDeps = _EMPTY_LIST; } return new ResolvedReflectiveFactory(factoryFn, resolvedDeps); }
步骤三 —— 合并已解析的Provider
const resolvedProviderMap = mergeResolvedReflectiveProviders(resolved, new Map()); export function mergeResolvedReflectiveProviders( providers: ResolvedReflectiveProvider[], normalizedProvidersMap: Map<number, ResolvedReflectiveProvider>): Map<number, ResolvedReflectiveProvider> { for (let i = 0; i < providers.length; i++) { const provider = providers[i]; // 从normalizedProvidersMap对象中获取key.id对应的ResolvedReflectiveProvider对象 const existing = normalizedProvidersMap.get(provider.key.id); if (existing) { // 如果当前的provider不是multi provider,则抛出异常 if (provider.multiProvider !== existing.multiProvider) { throw mixingMultiProvidersWithRegularProvidersError(existing, provider); } // 如果当前的provider是multi provider,则把当前provider的resolvedFactories // 列表中的每一项添加到已存在的provider对象的resolvedFactories列表中。 if (provider.multiProvider) { for (let j = 0; j < provider.resolvedFactories.length; j++) { existing.resolvedFactories.push(provider.resolvedFactories[j]); } } else { // 如果当前的provider不是multi provider,则覆盖已存在的provider normalizedProvidersMap.set(provider.key.id, provider); } } else { let resolvedProvider: ResolvedReflectiveProvider; // 如果当前的provider是multi provider,则创建一个新的ResolvedReflectiveProvider对象 if (provider.multiProvider) { resolvedProvider = new ResolvedReflectiveProvider_( provider.key, provider.resolvedFactories.slice(), provider.multiProvider); } else { resolvedProvider = provider; } // 在normalizedProvidersMap中保存已解析的ResolvedReflectiveProvider对象 normalizedProvidersMap.set(provider.key.id, resolvedProvider); } } return normalizedProvidersMap; }
步骤四 —— 生成ResolvedReflectiveProvider[]
// resolvedProviderMap的values,创建ResolvedReflectiveProvider[] Array.from(resolvedProviderMap.values()); /** * 基于一个类似数组或可迭代对象创建一个新的数组实例 * * arrayLike:转换成真实数组的类数组对象或可遍历对象。 * mapFn(可选):如果指定了该参数,则最后生成的数组会经过该函数的加工处理后再返回。 * thisArg(可选):执行mapFn函数时this的值。 */ Array.from(arrayLike[, mapFn[, thisArg]])
fromResolvedProviders()
该方法用于基于已解析的 providers 创建注入器。
static fromResolvedProviders(providers: ResolvedReflectiveProvider[], parent?: Injector): ReflectiveInjector { return new ReflectiveInjector_(providers, parent); }
fromResolvedProviders() 使用示例
@Injectable() class Engine {} @Injectable() class Car { constructor(public engine:Engine) {} } var providers = ReflectiveInjector.resolve([Car, Engine]); var injector = ReflectiveInjector.fromResolvedProviders(providers); expect(injector.get(Car) instanceof Car).toBe(true);
了解完 fromResolvedProviders() 方法的使用方式,接下来我们来重点分析一下 ReflectiveInjector_ 类。
ReflectiveInjector_ 类
ReflectiveInjector_ 类的属性
// 构造次数 _constructionCounter: number = 0; // ResolvedReflectiveProvider列表 public _providers: ResolvedReflectiveProvider[]; // 父级注入器 public _parent: Injector|null; // ReflectiveKey id列表 keyIds: number[]; // 依赖对象列表 objs: any[];
ReflectiveInjector_ 构造函数
export class ReflectiveInjector_ implements ReflectiveInjector { constructor(_providers: ResolvedReflectiveProvider[], _parent?: Injector) { this._providers = _providers; // 设置父级注入器 this._parent = _parent || null; const len = _providers.length; this.keyIds = new Array(len); this.objs = new Array(len); // 初始化keyIds列表和objs对象列表 for (let i = 0; i < len; i++) { this.keyIds[i] = _providers[i].key.id; this.objs[i] = UNDEFINED; } } } const UNDEFINED = new Object();
ReflectiveInjector_ 类的方法
ReflectiveInjector_ 类中的方法较多,我们只分析其中比较重要的方法,首先先根据方法的实现的功能进行分类:
用于创建ReflectiveInjector注入器
// 基于Provider列表并创建子注入器 resolveAndCreateChild(providers: Provider[]): ReflectiveInjector { const ResolvedReflectiveProviders = ReflectiveInjector.resolve(providers); return this.createChildFromResolved(ResolvedReflectiveProviders); } // 基于已解析的ResolvedReflectiveProvider列表,创建子注入器 createChildFromResolved(providers: ResolvedReflectiveProvider[]): ReflectiveInjector { const inj = new ReflectiveInjector_(providers); inj._parent = this; return inj; }
用于获取对象
// 获取当前注入器的父级注入器 get parent(): Injector|null { return this._parent; } // 获取token对应的依赖对象 get(token: any, notFoundValue: any = THROW_IF_NOT_FOUND): any { return this._getByKey(ReflectiveKey.get(token), null, notFoundValue); } // 根据ReflectiveKey及visibility可见性,获取对应的依赖对象 private _getByKey(key: ReflectiveKey, visibility: Self|SkipSelf|null, notFoundValue: any): any { // const INJECTOR_KEY = ReflectiveKey.get(Injector); if (key === INJECTOR_KEY) { return this; } // 判断该依赖对象是否使用@Self装饰器定义,表示从本级注入器获取依赖对象 if (visibility instanceof Self) { return this._getByKeySelf(key, notFoundValue); } else { // 使用默认的方式获取依赖对象 return this._getByKeyDefault(key, notFoundValue, visibility); } } // 从本级注入器获取依赖对象 _getByKeySelf(key: ReflectiveKey, notFoundValue: any): any { const obj = this._getObjByKeyId(key.id); return (obj !== UNDEFINED) ? obj : this._throwOrNull(key, notFoundValue); } // 使用默认的方式获取依赖对象 _getByKeyDefault(key: ReflectiveKey, notFoundValue: any, visibility: Self|SkipSelf|null): any { let inj: Injector|null; // 判断该依赖对象是否使用@SkipSelf装饰器定义,表示不从本级注入器获取依赖对象 if (visibility instanceof SkipSelf) { inj = this._parent; } else { inj = this; } // 从本级注入器获取依赖对象,若本级获取不到,则从父级注入器中查找 while (inj instanceof ReflectiveInjector_) { const inj_ = <ReflectiveInjector_>inj; const obj = inj_._getObjByKeyId(key.id); if (obj !== UNDEFINED) return obj; inj = inj_._parent; } if (inj !== null) { return inj.get(key.token, notFoundValue); } else { return this._throwOrNull(key, notFoundValue); } } // 获取keyId对应的对象,如依赖对象未创建,则调用_new()方法创建一个,然后保存到 // this.objs对象列表中 private _getObjByKeyId(keyId: number): any { for (let i = 0; i < this.keyIds.length; i++) { if (this.keyIds[i] === keyId) { // const UNDEFINED = new Object(); if (this.objs[i] === UNDEFINED) { this.objs[i] = this._new(this._providers[i]); } return this.objs[i]; } } return UNDEFINED; }
用于创建对象
// 创建依赖对象 _new(provider: ResolvedReflectiveProvider): any { // 判断是否存在循环依赖 if (this._constructionCounter++ > this._getMaxNumberOfObjects()) { throw cyclicDependencyError(this, provider.key); } return this._instantiateProvider(provider); } // 获取最大的对象个数 private _getMaxNumberOfObjects(): number { return this.objs.length; } // 根据已解析的provider创建依赖对象。若是multi provider则,循环创建multi provider对象。 private _instantiateProvider(provider: ResolvedReflectiveProvider): any { if (provider.multiProvider) { const res = new Array(provider.resolvedFactories.length); for (let i = 0; i < provider.resolvedFactories.length; ++i) { res[i] = this._instantiate(provider, provider.resolvedFactories[i]); } return res; } else { return this._instantiate(provider, provider.resolvedFactories[0]); } } // 根据已解析的provider和已解析的工厂创建依赖对象 private _instantiate( provider: ResolvedReflectiveProvider, ResolvedReflectiveFactory: ResolvedReflectiveFactory): any { // 获取对象工厂函数 const factory = ResolvedReflectiveFactory.factory; // 获取工厂函数所依赖的对象列表 let deps: any[]; try { deps = ResolvedReflectiveFactory.dependencies .map(dep => this._getByReflectiveDependency(dep)); } catch (e) { if (e.addKey) { e.addKey(this, provider.key); } throw e; } // 调用对象工厂函数创建依赖对象 let obj: any; try { obj = factory(...deps); } catch (e) { throw instantiationError(this, e, e.stack, provider.key); } return obj; }
用于获取工厂函数依赖对象
// 若通过@Optional装饰器定义该依赖对象,表示该依赖对象是可选的,当获取不到时返回null。 private _getByReflectiveDependency(dep: ReflectiveDependency): any { return this._getByKey(dep.key, dep.visibility, dep.optional ? null : THROW_IF_NOT_FOUND); }
以上就是本文的全部内容,希望对大家的学习有所帮助,也希望大家多多支持小牛知识库。
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